This invention relates to the preparation of high temperature superconducting oxide precursor materials, and, more particularly, to a coprecipitation technique that simultaneously forms and oxidizes the oxide.
One of the most important scientific advancements of recent years has been the discovery of materials that exhibit superconductivity at relatively high temperatures. In the superconducting state, a material has no electrical resistivity, and excludes magnetic flux lines. These characteristics can be used to great advantage in a variety of electrical and other types of devices. Until the recent discoveries, the maximum temperature at which superconductivity was observed (the "critical temperature" or T.sub.c of the material) was about 32K, which restricted the applications of the phenomenon to those wherein the material could be cooled to very low temperatures. The newly discovered types of high temperature nonmetallic superconductor materials in some cases have achieved the superconducting state above 77K, the boiling point of liquid nitrogen, and there exists the possibility of finding room temperature superconductors.
One of the important types of high temperature superconductors is complex oxides. For example, one class of the superconducting oxides is represented by the form M.sub.2(1-x) N.sub.2x CuO.sub.4-y, where y is less than x. M is a Group IIIA element or a rare earth, N is a Group IIA element, and y is as small a positive number as possible. An illustrative member of this class of superconducting oxides is La.sub.1.8 Sr.sub.0.2 CuO.sub.4. As used herein, a "high temperature oxide superconductor" is an oxide material of the form AO.sub.p, having a superconducting critical temperature of the material greater than about 60K.
The effectiveness of these oxides as superconductors is highly dependent upon their method of preparation. The superconducting oxides were first prepared by mechanically mixing the non-oxide elements, usually presented in the form of compounds such as oxides or carbonates, heating and sintering the mechanical mixture at a sufficiently high temperature to form a precursor material having a content of a phase that is superconducting except for an oxygen deficiency, and then oxidizing the mixture.
The mechanical mixing approach, also termed the solid-state reaction method, is somewhat ineffective in achieving a complete mixture. It may produce extraneous phases during sintering, with the result that the final oxide may contain non-superconducting regions and have a superconducting transition temperature below that otherwise expected.
In another approach, the crystallization method, the non-oxide components are provided in the form of nitrates, and co-crystallized to form a homogeneous mixture. One version of this procedure utilizes nitric acid as the solvent. The crystallized material is pyrolyzed to form the precursor material, which is then oxidized as for the solid-state reaction method. The crystallization method produces a more intimate mix of the components during the precipitation step than does mechanical mixing, with the result that pyrolysis does not lead to as high a content of extraneous phases. The crystallization method can, however, result in inhomogeneities within the co-crystallized material, because some possible nonsuperconducting reaction products can form in preference to the phase which, after oxidation, is superconducting.
With either method for preparing the precursor material, the final step is oxidation. During sintering, oxygen is lost to the atmosphere, and the sintered precursor material would not exhibit the desired superconductivity. The oxidation step is necessary to raise the oxygen content to the correct value, so that the final material has the required oxidation states and stoichiometry. The oxidation treatment is conducted by placing the precursor material into a furnace operating at a temperature sufficiently high to attain oxidation. In the case of the compound La.sub.1.8 Sr.sub.0.2 CuO.sub.4, for example, the heating and sintering temperature is about 900 C. and the oxidation temperature is about 500 C. The furnace atmosphere during oxidation is an oxygen-containing gas such as oxygen, air, or an oxygen/argon mixture. Oxidation has also been accomplished at lower temperatures using an oxygen plasma.
Although operable superconducting oxides have been produced by the approaches just outlined, they are time consuming and require multi-step processing at a range of elevated temperatures. During the various treatments, undesirable nonsuperconducting phases may form in preference to the superconducting oxide phase and be retained in the final product. There is therefore a continuing need for improved methods of preparing the superconducting oxides that are faster, more reliable, and avoid formation of extraneous phases. The present invention fulfills this need, and further provides related advantages.